1. Field of the Invention
The present invention relates to an ink tank used for an inkjet printing apparatus and the like.
2. Description of the Related Art
Examples of a printing apparatus using ink contained in an ink tank include an inkjet printing apparatus using an inkjet printing head capable of ejecting the ink. In addition, one of such inkjet printing apparatuses is a serial scan type apparatus having an inkjet printing head and an ink tank mounted on a carriage and configured to print an image on a printing medium while moving the carriage.
This serial scan type inkjet printing apparatus includes a carriage capable of mounting an inkjet printing head and an ink tank for supplying ink to the printing head. In printing, the apparatus causes the printing head to eject ink droplets through fine ejection openings provided in the printing head while moving the carriage relative to the printing medium. Thus, the ink droplets land on the printing medium and a desired image is printed.
Dye inks each using a dye as a colorant have been used chiefly as inks for inkjet printing heads. However, it is difficult to cause printed matters with dye inks to exhibit as high performances as those required for application such as an outdoor printed display for which light resistance and weather resistance are considered important. For this reason, instead, pigment ink using a pigment as a colorant has been put into practical use. Nevertheless, pigment particles in pigment ink inevitably are settled down in an ink tank because the pigment is not of a solution type, but of a dispersion type.
For instance, in a case where an ink tank is left unused while mounted on an inkjet printing head for a long period of time, pigment particles in the ink are gradually settled down in the ink tank. For this reason, the concentration gradient of the pigment particles occurs from the bottom portion toward the uppermost potion of the ink tank. As a result, ink in the bottom portion of the ink tank forms an excessively-dense color layer due to a higher concentration of the pigment particles, whereas ink in the uppermost portion of the ink tank forms an excessively-light color layer due to a lower concentration of the pigment particles.
Let us consider a case where an ink tank configured to lead out ink from its bottom portion is stored at a certain posture (with its bottom portion faced down in the vertical direction) for a long period of time. When ink led out of such an ink tank is supplied to a printing head, the ink in the layer with a higher concentration of pigment particles is supplied first, thereby printing an image with an excessively-dense color. In other words, a visible difference in printed density may occur between printed images formed in an early stage and a later stage of the ink tank use. This phenomenon is conspicuous particularly in a color printing operation for printing a color image by use of gradations of color.
To solve these problems, each of Japanese Patents Laid-open Nos. 2004-216761 and 2005-066520 discloses a configuration in which an agitating member (agitating body) is provided in an ink tank. In the configuration, ink is agitated in the ink tank by moving the agitating body with an inertial force occurring due to the reciprocating motion of the carriage.
Specifically, Japanese Patent Laid-open No. 2004-216761 discloses an ink tank having a swingable agitating body thereinside. The center of the swing of the agitating body is set almost in the center of the inside of the ink tank in a movement direction of the carriage. Accordingly, this agitating body swings similarly in one and the other directions in response to the reciprocating motion of the carriage. Furthermore, Japanese Patent Laid-open No. 2005-066520 discloses an ink tank having an agitating body thereinside, the agitating body being swingable with elastic deformation. This agitating body is hung from a substantially central portion of the upper inner surface of the ink tank. This agitating body also swings similarly in one and the other directions in response to the reciprocating motion of the carriage. In addition, Japanese Patent Laid-open No. 2005-066520 describes another configuration in which an ink tank has an agitating body thereinside, the agitating body being freely movable on the bottom surface of the ink tank. This agitating body freely moves on the bottom surface of the ink tank due to the reciprocating motion of the carriage.
Moreover, Japanese Patent Laid-open No. 2007-230189 discloses a configuration in which: agitating members are provided in the inside of an ink containing chamber of an ink tank, the agitating members agitating ink in the ink containing chamber; and an ink leading-out port is provided in the ink containing chamber at a position higher than its bottom portion that is situated lowermost when the ink tank is placed at a posture for use. Because the ink leading-out port is placed higher than the lowermost portion, the sedimentary ink in a lowermost layer with a higher specific gravity is prevented from entering the ink supply port of a printing head. Each agitating member is configured to have an end-side portion rotatably supported by a supporting part provided in the ink containing chamber; and a support point for the rotation linearly movable along the supporting part.
However, the ink tanks respectively described in Japanese Patents Laid-open Nos. 2004-216761 and 2005-066520 have the following problems. First, in the ink tank disclosed in Japanese Patent Laid-open No. 2004-216761, the agitating body swings similarly in one and the other directions about the substantially central portion of the inside of the ink tank. For this reason, to enhance the agitating performance of the agitating body by widening the swingable range of the agitating body, the width of the ink tank needs to be enlarged in the movement direction of the carriage. However, since multiple ink tanks are mounted on the single carriage in the movement direction of the carriage in many cases, the ink tank inevitably has to be formed with a relatively small width. Thereby, the swingable range of the agitating body cannot be widened, and the ink flow caused by the swing of the agitating body is small. To fully agitate the ink, time for agitation needs to be extended by increasing the number of times the carriage reciprocates.
On the other hand, in ink tank disclosed in Japanese Patent Laid-open No. 2005-066520 including the agitating body hung from the substantially central portion of the upper inner surface of the ink tank, the agitating body swings similarly in one and the other directions about the substantially central portion of the inside of the ink tank. To enhance the agitating performance of the agitating body by widening the swingable range of the agitating body, the width of the ink tank in the movement direction of the carriage needs to be enlarged as in the ink tank disclosed in Japanese Patent Laid-open No. 2004-216761. In this respect, the ink tank described in Japanese Patent Laid-open No. 2005-066520 has the same problem as the ink tank described in Japanese Patent Laid-open No. 2004-216761. In addition, in a case where the acceleration of the carriage is set larger to elastically deform the agitating body to a larger extent, a larger and more expensive driving source (a motor or the like) may be needed for the carriage, and the printing apparatus may cause larger vibrations. In the ink tank disclosed in Japanese Patent Laid-open No. 2005-066520 including the agitating body freely movable on the bottom surface of the ink tank, the agitating body has a problem of having a poor capability of agitating an upper portion of the ink in the ink tank because the agitating body is away from the upper portion of the ink.
These problems with the ink tanks disclosed in Japanese Patents Laid-open Nos. 2004-216761 and 2005-066520 are also clear from a viewpoint of configurations of a generally-used ink tank and printing apparatus.
In general, a width and length of ink tanks mounted on a carriage are set to enhance the usability in attaching and detaching the ink tank. Specifically, the width of the ink tank in the movement direction of the carriage is set relatively small, and the length of the ink tank in a conveyance direction of a printing medium which crosses over the movement direction of the carriage is set relatively large. For this reason, it is difficult to set the agitating body to be largely displaceable in its displacement direction that is in parallel with the movement direction of the carriage. As a result, the amount of displacement of the agitating body is so small that strong flow of the ink cannot be caused. This limits the agitating body's agitating efficiency of the ink, and requires too much time to fully agitate the ink in the ink tank. For instance, in a case where pigment particles contained in the ink in the ink tank are settled down because the printing apparatus has carried out no printing operation for a long period of time with the ink tank being mounted on the carriage, the carriage has to be reciprocated for a long time before starting a printing. This increases ‘warm-up’ time before the printing apparatus can start the printing operation. Particularly, in a case where the particle size of the pigment contained in a pigment ink is large, or in a case where the specific gravity of pigment particles is large, the pigment particles are settled down quickly. Even if the ink tank is left unused for several days, the ink in the ink tank may have a concentration distribution having adverse affects on printed images. In this case, the ink needs to be agitated every several days, and the printing apparatus can start no printing operation each time the ink is agitated.
FIG. 14 shows a concentration gradient of ink in a height direction of an ink tank. When no pigment is settled down, the concentration of the ink is homogeneous, and the ink has the same concentration in any height. However, once the ink is stored beyond a certain length of time, as shown by a curve B in FIG. 14, the concentration of the ink becomes lower in a higher portion in the tank, and gradually becomes closer to an initial concentration as the height becomes lower. The concentration of the ink in the middle of the ink tank is substantially equal to the initial concentration. On the other hand, the concentration of the ink becomes higher in a lower portion in the tank. In a portion closer to the lowermost portion (bottom portion) of the ink tank, particularly, the concentration of the ink changes (increases) suddenly. In general, the viscosity of the ink becomes higher as the concentration of pigment particles becomes higher. For this reason, the ink in the lowermost portion has higher specific gravity and viscosity than the ink in the other portions, and accordingly forms a layer which has properties largely different from those of the other portions.
To address this problem, in the ink tank described in Japanese Patent Laid-open No. 2007-230189, the ink leading-out port in the ink containing chamber is placed higher than the lowermost portion of the ink tank. Thereby, a sedimentary pigment settled down in the lowermost portion of the ink tank is prevented from moving into the ink supply port.
However, it is known that a sedimentary pigment thus settled down behaves like a viscous fluid, more specifically, has properties in which, once starting the movement, such sedimentary pigment moves while drawing its surrounding sedimentary pigment. For this reason, if the ink leading-out port in the ink containing chamber is formed in a horizontal surface when the ink tank is placed at a posture for use, most of the sedimentary pigment settled around the ink leading-out port may move into the ink leading-out port and further to the inside of the ink supply port, so that a desired effect cannot be obtained.
Furthermore, the placing of the ink leading-out port in a position higher than the bottom surface of the ink tank means that a portion in which the ink leading-out port is formed projects from the bottom surface of the ink containing chamber. This configuration, however, has the following problem in the case where the ink tank includes a flexible member for changing the volume of the ink containing chamber, and is configured to change the volume of the ink containing chamber in order to alleviate the increase in a negative pressure with the leading-out (consuming) of the ink while taking no atmosphere thereinto. In this case, as the amount of remaining ink in any of the ink tanks becomes smaller, the ink leading-out port may be covered by the flexible member, and the amount of ink which can be led out or supplied may vary from one ink tank to another.
Moreover, the agitating body is able to exert higher agitating performance when being placed in a position closer to the lowermost surface of the ink containing chamber in which a sedimentary pigment is settled down. However, depending on the height of a member forming the ink leading-out port, the agitating body has to be configured to avoid an interference with the member, so that the agitate body cannot be placed in a position where it can agitate effectively, or may be formed in a complicated shape. Consequently, the agitating performance of the agitating body may decrease.